The present invention relates to the evaluation of formations that are penetrated by a borehole in the search for hydrocarbon deposits. Many hydrocarbon deposits are found in fractured reservoirs as contrasted to sand or other types of reservoirs. The presence of a fractured reservoir is difficult to detect from measurements made in the borehole and valuable formations are missed. The difficulty arises from the fact that fractured reservoirs exhibit very small differences in most logging parameters from formations immediately adjacent the fractured formations. Also, since most wells are drilled using drilling mud, the open fractures tend to be sealed by the drilling mud, thus making their actual detection difficult. In addition to the above problems there is a possibility that the borehole does not actually intersect a fracture and that the fractures are located at some distance from the borehole wall. If the fractures could be detected the formation could be produced by inducing fractures in the formation to establish fluid communication between the natural fractures and the borehole.
Several methods have been proposed in an attempt to evaluate fractured formations to determine if fractures are present and if present, if they contain hydrocarbons in commercial quantities. One method consists of using various electrical logs in combination with conventional acoustic logs to detect differences between the various formations penetrated by the borehole and correlate the differences with core samples taken from wells in the same field. A further method is described in U.S. Pat. No. 4,130,816. This patent describes a method and apparatus using four acoustic transducers which are equally spaced circumferentially in a horizontal plane. The apparatus includes means for holding transducers in close proximity to the wall of the borehole with two transducers being used as transmitters and two as receivers. The transducers produce acoustical pulses which travel circumferentially along the borehole wall from the transmitter to the receiver. If a fracture is present, the amplitude of the received acoustic pulse will be reduced over the acoustic pulse received in the absence of fractures. While the apparatus has had success in locating fracture formations, it does have several limitations. For example, the transducers are not in direct contact with the borehole wall and thus they waste considerable acoustic energy in the fluid filling the borehole that interferes with the acoustic signals of interest. Further, since the tool utilizes acoustic waves which travel circumferentially around the borehole wall the depth of penetration is very shallow and fractures which do not intercept the borehole will not be detected.